Pathogenic mutations in the tau gene (MAPT) are linked to the onset of tauopathy, but the A152T mutation is unique in acting as a risk factor for a range of disorders including Alzheimer?s disease (AD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and dementia with Lewy bodies (DLB). As an unconventional approach to investigate the role of tau in neurodegeneration, we reasoned that understanding how the A152T variant modulates risk of AD and related disorders could reveal a common disease mechanism(s), uncovering novel strategies to increase resilience to tau toxicity and modify disease phenotypes in patients. Given the introduction of a new potential phosphoepitope, we questioned whether the A152T variant might impact disease risk through altered phosphorylation of tau on either T152 or the neighboring T153 residue. A series of novel antibodies were generated to test this idea, which revealed significant accumulation of soluble tau species hyperphosphorylated on T153 (pT153) in postmortem brain tissue from A152T carriers compared to noncarriers, as well as in mice expressing A152T-AAV. Therefore the current project will investigate the overall hypothesis that the A152T variant modulates disease risk through enhanced accumulation and increased solubility of pT153-positive tau, which subsequently primes tau for downstream pathological phosphorylation events and is critical for tau-mediated toxicity. Of note, phosphorylation on T153 and tau?s other serine/threonine-proline motifs has been shown to be required for tau toxicity, although the extent to which pT153 contributes to tau toxicity remains untested. In elucidating the pattern of pT153 deposition throughout the brain in A152T carriers and noncarriers, the proposed studies will determine if pT153-positivity coincides with neurodegeneration. Using site-directed mutagenesis and somatic brain transgenesis, we will determine whether pT153 is required for tau toxicity in vivo. Finally, incorporating rapidly evolving technology that is enabling acquisition of global gene expression profiles at the single cell level, we will assess whether expression of the A152T variant differentially impacts the transcriptome of individual cell populations. We anticipate that in uncovering the mechanisms by which A152T influences risk of tauopathy, and deciphering the involvement of pT153 in tau toxicity, the current project could identify novel approaches to block tau-mediated neurodegeneration in AD and related disorders.